1. Field of the Invention
The present invention relates to a hydraulic cylinder.
2. Description of the Related Art
In general, the hydraulic cylinder includes a piston, a cylinder, and a seal member for sealing an annular clearance between them. In the hydraulic cylinder, a piston is inserted into a shaft hole formed in the cylinder while leaving a minute clearance between an inner circumferential face of the shaft hole and the piston.
In a hydraulic cylinder such as that used for a breaker in which the piston and the cylinder relatively reciprocate at high speed and which is used in an environment of high hydraulic pressure, cavitation occurs in the minute clearance.
If the cavitation occurs, a cavitation shock wave or the like due to collapse of a bubble instantaneously generates extremely high pressure around a position where the cavitation has occurred. As a result, if the cavitation occurs near a lip of a seal member mounted into an annular mounting groove formed in an inner periphery of the shaft hole, vermiculate chips occur at portions near a tip end of the lip and sealing performance reduces.
In a technique disclosed in Japanese Patent Application Laid-Open No. 2008-2571, for example, a notch is formed in a portion of a mounting groove into which a seal member is mounted and near a tip end of a lip to thereby increase a distance from a position where cavitation occurs to the seal member. Such related art will be described with reference to FIG. 6. FIG. 6 is a schematic sectional view of an area of a related art hydraulic cylinder where the seal member is provided.
As shown in the drawing, in the hydraulic cylinder, a piston 200 is inserted into a shaft hole in a cylinder 300. A packing 100 that is the seal member is mounted into the annular mounting groove 301 formed in an inner periphery of the shaft hole of the cylinder 300. In the related art, oil is sealed in on a left side in the drawing and gas is sealed in on a right side in the drawing. Hereafter, the left side in the drawing will be referred to as an oil side (O) and the right side in the drawing will be referred to as a gas side (G). In the related art, the packing 100 includes a packing main body 101 and a backup ring 102 for preventing the packing main body 101 from protruding into a minute clearance S at an inner circumferential end portion on a low-pressure side.
In the related art, a small-diameter groove 302 (notch) adjacent to the mounting groove 301 is formed on a side of a tip end (oil side (O)) of a lip 101a of the packing main body 101. As described above, the cavitation occurs in the minute clearance S and the groove 302 increases a distance from a position where the cavitation occurs to the packing 100. In this way, it is possible to suppress reduction in sealing performance caused by the cavitation.
In the hydraulic cylinder according to this related art, however, the groove 302 having a relatively large clearance is formed on the side of the tip end of the lip 101a of the packing main body 101 and therefore, part of or the whole lip 101a may be fitted in the groove 302. This may cause damage to the lip 101a such as a chap at a tip end of the lip 101a and a chip of a portion of the lip 101a. Therefore, it is difficult to provide the groove 302 as measures against the cavitation as in the related art depending on specifications.